What facten said, plus 1, regarding Brent Jessee.
Something VERY important, when tubing a preamp, is finding the LEAST noisy tubes possible.
Brent uses my favorite method of weeding out noisy tubes. Actually listening to them, as he outlines (a video), on the page that Facten linked.
I couldn’t agree more, as regards the sound of the Siemens E88CC variants, having used their early 60’s, grey shield, external date coded, CCa’s, in my ARC SP-9 and SP-14 pres, decades back as well as the six, in my current BAT VK-D5.
I’ve also a few matched pairs of Siemens E188CC and E88CC’s, of the same era and construction, for backup. Their presentation varies precious little, from the CCa, allowing whatever they're fed, to proceed as recorded.
If your aural taste runs more toward warmth: (the best Siemens and Telefunkens of bygone years, being some of the most uncolored valves extant) you might add a pair of early (same era, early 60’s) Amperex, like their NAVY 6922 or PQ 7308, which will not detract from what the Siemens deliver, in any way (been there/done that, just a tiny bit too warm for my palate).
Give Brent a call. He won’t steer you wrong!